Horticulture Principles and Practices

Before the seed or

Before the seed or product becomes available for sale, it goes through a certification process. Certification is conducted by certifying agencies according to prescribed guidelines for the crop or species. For seed products, the breeder retains the most authentic version of the cultivar called the breeder seed. Some of the seed is increased by a contracted producer to obtain the foundation seed. This seed is further increased by farmers under contract to yield the registered seed. Registered is further increased to obtain certified seed for sale to farmers. These are called seed classes. For certification, the agency inspects the variety in the field according to the guidelines for the crop. The breeder is also required to furnish specific product information including the history and origin of the cultivar and documentation of the processes of evaluation and plans for maintenance of the various seed classes. The seed has to be submitted for testing (seed testing) to determine viability or germination percentage, purity, vigor, seed health, and noxious weed seed. Before being offered for sale, the bags are tagged, as described in Chapter 9. 6.16 BREEDING ASEXUALLY PROPAGATED SPECIES As previously indicated, variation is the lifeblood of plant breeding. Recombination is the primary source of variation for species that reproduce sexually. For species that do not reproduce sexually, variation can be generated through other means such as inducing artificial mutations. Mutations have been used, not only in asexually reproducing species but also in sexually producing species, to develop numerous commercial species of fruit trees, ornamentals, and other species. Tissue culture and other biotechnological techniques that can be used are discussed next. 6.17 BREEDING SEEDLESS FRUITS Seedlessness in fruits is a desirable trait in horticulture breeding. Seedless fruits are more convenient to eat because there are not seeds to spit out. Seedless cultivars are commercially available for fruits such as watermelon, grape, orange, and strawberry. The conventional method of breeding seedless fruits is the use of triploid hybrids. To obtain a triploid, a tetraploid (4x) parent is crossed with a diploid (2x) line. In watermelon, for example, the tetraploid is always the female parent (4x = 44; 2x = 22). The reciprocal cross, with the female as male parent, does not produce seed (Figure 6–3). The triploid resulting from this cross (3x = 33) is female-sterile and hence the fruit is seedless. Select diploid line Produce tetraploid plants Colchicine treatment; e.g., shoot apex treatment FIGURE 6–3 Breeding seedless watermelon. Develop tetraploid lines Evaluate tetraploid lines Triploid development Female 4x = 44 (gray rind) x Male 2x = 22 (striped rind) Evaluate triploid 3x = 33 (striped rind) (female/male sterile 6.17 Breeding Seedless Fruits 203

Because the triploid is also male-sterile, growers of seedless watermelon must plant rows of diploid lines as pollinators for stimulation of fruit formation. 6.18 BIOTECHNOLOGY IN PLANT BREEDING So far, we have discussed the concepts and methods of conventional plant breeding. This section is devoted to discussing the principles and concepts of nonconventional breeding at an introductory level. Recombinant DNA A hybrid DNA molecule produced in the laboratory by joining pieces of DNA from different sources. 6.18.1 WHAT IS BIOTECHNOLOGY? Biotechnology may be broadly defined as the use of techniques based on living systems to make products or improve other species. This definition would include the use of microbes to make products by the age-old process of fermentation. However, a narrower definition would restrict the term to the genetic manipulation of organisms for specific purposes. Again, this definition would include classic plant breeding by crossing. Another term is used to distinguish between the levels of genetic manipulation—indirect manipulation of plant genetics by conventional methods at the whole organism level using methods like crossing, and direct genetic manipulation at the molecular level (Figure 6–4). The term genetic engineering is used to describe the genetic manipulation of organisms at the molecular level, directly involving the DNA. Scientists, using the revolutionary technology of recombinant DNA (rDNA), are able to transfer genes from any organism to another. What makes this radical gene transfer possible is the fact that DNA is “universal.” Regardless of source, all DNAs obey the same rules and are chemically identical. So, it is possible to transfer DNA from an animal and insert it into a plant! This, theoretically, removes all biological barriers, reducing all life into just one humongous gene pool. The controversy with genetic engineering primarily stems from this radical approach to genetic manipulation. The term molecular breeding is used to describe the application of a variety of tools for manipulating the DNA of plants, which may or may not involve rDNA, to improve them for specific purposes. Even though crossing two different parents produces new recombinants in the segregating population, the term recombinant DNA is reserved for the union of DNA segments of different biological origins. A cultivar developed by genetic engineering protocols is called a transgenic cultivar or a genetically modified (GM) cultivar. Generally, an organism developed by rDNA procedures is called a genetically modified organism (GMO). FIGURE 6–4 Levels of eukaryotic organization. Whole plant Organs Tissues Cells Organelles Macromolecules 204 Chapter 6 Breeding Horticultural Plants